GROUND WATER IN NORTH-CENTRAL TENNESSEE

SPRINGS ' 91
formation, which are known to exist only locally along the Highland
Rim escarpment in Williamson County, on the west side of the Nash­
ville Basin. Several of the springs that constitute the municipal
supply of Franklin (Nos. 366, 380, 383, and 384, pp. 218-219) are
typical of the springs that issue from this horizon. Except at these
two horizons, contact springs are not especially numerous at the out­
crop of any one stratigraphic member, although many springs issue
just above clay beds that separate layers of limestone and near the
base of the zone of weathering in rocks of all types. Except within
very small areas, however, the stratigraphic horizon of these springs
is largely a matter of local variations in the texture of the rocks.
The seepage springs of north-central Tennessee are generally small,
and many are intermittent. Few of those that issue from the Chat­
tanooga shale yield more than 1 gallon a minute in dry seasons, and
most of those that issue from other rocks do not yield more than 25 gal­
lons a minute. Moreover, their yield is likely to vary from season to
season, perhaps greatly, although the change in discharge is gradual.
Under favorable conditions the discharge of several such springs may
be combined into a reliable supply of considerable magnitude, such as
the municipal supply of Franklin, Williamson County. This supply
comprises 34 springs on the Highland Rim escarpment about 12 miles
west of the city, the smallest of which discharges about 3 gallons a
minute in dry seasons and the largest about 28 gallons a minute.
The springs that issue from alluvium or coarse hill wash above bed­
rock constitute a reliable source of water throughout the dissected
portions of the Highland Rim plateau, especially along the eastern
slope of the Tennessee River Valley. The largest of these generally
issue from extensive beds of coarse detritus that have been deposited
by the intermittent streams where their gradients flatten at the base of
the dissected upland. Typical springs of this class are Nos. 161 and
174 of Humphreys County (pp. 161-162), No. 224 of Dickson County
(p. 147), and No. 252 of Cheatham County (pp. 129-130). If the
alluvium is well assorted its permeability is high, so that, given an
adequate volume of stored water, a spring may have a relatively
large and only moderately variable yield. Many discharge more
than 25 gallons a minute in dry seasons and some more than 100
gallons a minute.
The water that supplies the seepage springs does not, as a rule,
penetrate far below the surface, so that it is nonthermal, and usually
its temperature is approximately equal to the mean annual air temper­
ature of the district. In north-central Tennessee the mean annual
temperature is between 58° and 60° F. In a spring of very small
yield, however, the temperature of the water is likely to be variable
and to follow the diurnal variations in air temperature.
The seepage springs differ greatly in the chemical character of their
water. Those that issue from thoroughly weathered and leached